The present invention relates generally to grinding wheel drive systems and more particularly to a grinding wheel drive system in which the grinding is performed at a constant power level.
It has long been known that improved grinding results can be obtained by maintaining the peripheral or surface speed of the grinding wheel constant. Because the wheel diameter decreases with use, these grinding systems require a variable speed drive so that as the diameter of the wheel gets smaller the rotational speed is increased proportionally. More recently it has been learned that in certain applications it is also desirable to maintain the grinding at a constant power (e.g., watts or horsepower) level, proportional to the desired surface speed.
That a constant surface speed alone will not necessarily achieve a constant power and grinding level becomes obvious when it is realized that the power put into a grinding operation is a function not only of the wheel surface speed but also the force or pressure of the contact between the wheel and the workpiece. The maintenance of constant power grinding would present no problem if one could provide constant surface speed on the wheel and constant pressure between the wheel and the workpiece. Many methods of maintaining constant surface speed are known, but the difficulty occurs in finding a suitable means of measuring pressure. The use of load cells, scales, etc., is not always feasible since in many grinding machines, the weight of the supporting means is many times the pressure it is desired to measure. On some types of grinding machines, the armature current of the motor which rotates the grinding wheel has been used to measure the pressure. This is quite satisfactory since the armature current is directly proportional to the pressure on the grinding wheel. However, by virtue of the changing torque arm, the armature current is also inversely proportional to the radius of the grinding wheel for a given pressure, such that, as the wheel wears down, the current corresponding to a given pressure changes according to a definite law. Thus, if the grinding wheel is moved normal to the work surface to maintain constant armature current, grinding power will be constant only so long as constant surface speed is maintained and wheel diameter does not change.
It is also known that certain motors will operate at constant horsepower output under certain conditions. The d.c. shunt motor with separate field excitation is an example of such a motor when the motor is operated at above its rated or based speed. The use of such a motor solely in this mode is not, however, entirely satisfactory for all grinding applications. Speeds above the base speed of such a motor may be too high for the desired grinding operation and the use of speed reduction apparatus such as gears is not a sufficiently accurate or satisfactory way of achieving the speed reduction. In addition, the d.c. shunt motor with separate field excitation has a tendency to exhibit stability problems when operated in the constant horsepower region above rated base speed such that even though there are various methods for lessening this problem, the overall control is not as completely satisfactory as desired.